Electron probe employing three secondary emission detectors whose outputs are combined to minimize error

ABSTRACT

In electron beam apparatus in which an electron beam is caused to impinge on a specimen surface and the backscattered electrons are detected and used to produce an image-giving information about the surface, instead of using a single detector, which cannot distinguish between changes in signal due to material and changes due to topography, and instead of using two detectors which can eliminate changes due to material but only show topographical features in certain directions, it is proposed to use at least three detectors, symmetrically arranged, and to combine their signals in such a way as to allow observation of topography without regard to angular position or with regard to a direction of apparent illumination that is variable at will.

United States Patent I NM l l Inventors Alistair John Campbell;

Andrew DtG. Stewart. both of Cambridge,

ELECTRON PROBE EMPLOYING THREE SECONDARY EMISSION DETECTORS WHOSE OUTPUTS ARE COMBINED TO MINIMIZE ERROR l 1 Claims, 4 Drawing Figs.

us. Cl 250/495 PE,

250/495 P Int.Cl G0ltl/l6 FieldofSearcli 250/495,

49.5 A, 49.5 P, 49.5 PE

References Cited Primary Examiner-1ames W. Lawrence Assistant Examiner-C. E. Church AttorneyScrivener, Parker, Scrivener and Clarke ABSTRACT: In electron beam apparatus in which an electron beam is caused to impinge on a specimen surface and the backscattered electrons are detected and used to produce an imagegiving information about the surface, instead of using a single detector, which cannot distinguish between changes in signal due to material and changes due to topography, and instead of using two detectors which can eliminate changes due ,to material but only show topographical features in certain directions, it is proposed to use at least three detectors, symmetrically arranged, and to combine their signals in such a way as to allow observation of topography without regardto angular position or with regard to a direction of apparent illu mination that is variable at will.

PATENTEU AUG 3 I97! ELECTRON PROBE EMPLOYING THREE SECONDARY EMISSION DETECTORS Wll-llOSlE OUTPUTS ARE COMBINED TO MINIMIZlE ERROR This invention relates to electron beam apparatus in which an electron beam is caused to impinge on the surface of a specimen and the backscattered electrons which emerge from the specimen are observed by appropriately placed detecting instruments.

The purpose of this analysis is to observe the nature of the surface of the specimen, in particular the topography and the distribution of the various elements present, for example the distribution of inclusions in a specimen of steel or cast iron.

The degree of back scattering is dependent on the atomic number of the element scanned, and by synchronizing a display on a cathode ray tube screen or on a two-coordinate plotter with the scanning of the electron probe over an area of the specimen surface it is possible to produce a picture of the element distributions.

However, the distribution of the backscattered electrons depends not only on the distribution of the elements but also on the topography of the specimen surface. Accordingly it has recently been proposed to distinguish between the influences of these two factors by providing two electron detectors spaced apart symmetrically on opposite sides of the electron beam and both receiving backscattered electrons from the same scanned spot on the specimen surface. Their outputs are added to produce a signal dependent on atomic number that is at least partially independent of topography and are subtracted to produce a signal that depends on the topography but is independent of atomic number.

Such a solution is, however, only partial as it is markedly directional, that is to say, it only detects topographical features whose general direction runs perpendicular to that plane which contains the electron probe and the two detectors.

It is analogous to viewing a landscape from above, with the illumination nearly parallel to the plane of the landscape and coming only from one side. Consequently, prominent topo graphical features, if they run parallel to the direction of apparent illumination, can be missed altogether in the topographical picture and cannot be eliminated from the atomic number picture.

It is not desirable to rotate either the specimen or the detectors (in order to obtain a second topographical picture show ing the missing detail) since firstly it is difficult to provide a sufficiently precise rotation about the point of impact of the beam and secondly the structure of a fragile or volatile specimen may change in the interval between obtaining the first and second topographical pictures.

The objects of the present invention is to provide means for detecting topographical features which run in any direction simultaneously and without moving either the specimen or the detectors. A further object is to provide improved means for displaying atomic number distributions.

According to the invention we now propose to provide at least three detectors for backscattered electrons and to connect their outputs to a mixing circuit in such a way as to produce one or more signals which are independent of atomic number and which are used to display all the topographical features ofa specimen. ln a first embodiment of the invention, the outputs of the detectors are connected to one or more mixing circuits which allows these outputs to be combined in a manner such that the topographical display, derived from the circuit, has the appearance of being illuminated from substantially any desired direction at will. In a second embodiment of the invention, the outputs of the detectors are connected to a mixing circuit which combines these outputs in a manner such that the display, derived from the circuit, shows all the topographical features of the specimen, whatever their direction, but does not distinguish between hills and valleys.

It will be understood that as in the known arrangement, the output of all the detectors can be added to produce a signal that is dependent on atomic number but independent of topography and, in fact, this should be achieved with much greater success than in the known arrangement with only two detectors.

The invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic perspective view showing the use of three detectors;

FIG. 2 is a plan view corresponding to FIG. 1;

FIG. 3 is a diagram to assist in an understanding of the mathematical relationships; and

FIG. 4 is aplan view showing the use of four detectors.

Referring first to FIG. 1, a finely focused electron beam or so-called probe B is caused in a known manner to impinge on the surface of a specimen S of which the nature, both chemical composition and surface quality or topography, are to be analyzed. The impingement of the electrons causes the emission of backscattered electrons, which may be partially secondary electrons and partially the impinging electrons reversed in direction. These are picked up by electron detectors.

The electron beam Bwill normally be scanned laterally in a raster in two mutually perpendicular directions parallel to the surface of the specimen, so as to cover a small selected area, and the outputs of the detectors are fed to the brightness control of a cathode-ray tube, of which the beam is scanned in synchronism with that of the beam B, so that there is displayed on the screen of the cathode ray tube a picture of the distribution of the electron backscatter behavior over the scanned area.

Where the surface of-the specimen is perfectly flatand perpendicular to the electron beam then the distribution of the backscattered electrons is, for reasons of symmetry, uniform about the axis of the beam 8, and the degree of back scattering is dependent on the atomic number, being greater for the elements of higher atomic number. However, where the surface is undulating, as by the formation of microcrystals having exposed surfaces in planes which are inclined to the overall plane of the whole surface, then the backscattered electrons will tend to be distributed asymmetrically.

Whereas such an inclined surface would be detectable by the use of two detectors lying on opposite sides of a plane containing the beam and parallel to the line of intersection of that inclined surface with the-overall specimen surface, such an arrangement would not detect an inclined surface intersecting the overall specimen surface in a line perpendicular to that plane.

We overcome this by the use of three or more detectors. In FIGS. 1 and 2 we show the use of three detectors D D and D distributed uniformly apart as viewed in the direction of the impinging electron beam.

The electrical output signals obtained from the three detectors are fed to a mixing circuit M. Where one is interested in knowing the nature of the elements present in the region under analysis these three outputs are simply added, and the resultant signal is dependent on atomic number, but is independent of the surface topography. Where the surface topography is the important thing, one combines the three outputs (1 ,11 and d to produce an output D in accordance with the equation:

where 0 is the angle which the projection of the line to the detector D, makes on the surface of the specimen with an arbitrary line in the specimen surface which we can aptly describe as the direction of apparent illumination. This is illustrated in FIG. 3.

It will be seen that, where the direction of apparent illumination coincides with the direction of the detector D,,, this reduces to:

given by:

-1 2 d; cos

The electrical circuits necessary for combining the signals from the detectors will be well understood by those skilled in the art and there is no need to illustrate them here. In addition to providing a selector switch that allows the choice of either plain addition of the signals from all the detectors or the combination in accordance with the equation above, we also preferably include a knob calibrated in degrees of angle and allowing the direction of apparent illumination to be swung round at will through at least 90 and possibly even through 360, by variation of the angle in the equation. In this way, on a given specimen, one can, without rotating the specimen or the scanning system, examine rapidly and easily those sur- 7 face irregularities which extend predominantly in any selected direction, and one can ensure that no irregularities are missed, as could happen where the direction of apparent illumination is fixed.

In an alternative arrangement, instead of using the equations given above, we combine the signals in a different way. We ascertain the arithmetic mean of all the signals and then subtract from this all the individual signals in turn, and add together the resulting values, regardless of sign. Thus, where there are three detectors we obtain an output:

Alternatively, a similar result is obtained by adding together the squares of the values obtained by subtracting each of the signals in turn from their mean value. This produces an output D which, in the case of three detectors, is represented by:

Either of these signals l) or 0 will give a topographical display which, like the earlier version, is independent of the atomic number of the elements being scanned, but in this version there is no distinction between hills and valleys. The signals D and I) may be obtained equally well where there are more than three detectors but cannot be obtained with fewer than three.

it will be understood that an electron-probe microanalyzing instrument can be equipped with switches allowing the immediate selection of any ofthe signals D, D or I) as well as a knob controlling the angle of apparent illumination and a switch allowing the selection of the atomic number signal (d +d,+d,) instead.

The proposal according to the invention, to use three or more detectors has an important advantage over the use of two detectors, even where their outputs are simply being added together to produce the atomic number signal. Where there are only two detectors a scratch or surface irregularity extending in a direction parallel to the line joining the two detectors will not be apparent but may have an equal effect on the signals in both detectors and thus introduce an apparent variation in composition that is not actually present. This is eliminated by the use of three or more detectors.

We claim:

1. Electron beam apparatus comprising means for causing an electron beam to impinge on the surface of a specimen placed in the path of said beam, an array ofn electron detectors, n being at least three, spaced symmetrically around the path of said beam in positions to pick up resultant backscattered electrons from said specimen surface, said detectors producing electrical signals dependent on the backscattered electrons picked up, means for causing relative lateral scanning between said beam and said specimen surface, and

an electric mixing circuit, said mixing circuit being arranged to produce an output where d,,, d, d,, are the electric signals from said detectors, and 6 is the angle between an arbitrary direction lying in a plane normal to the beam and the projection, onto said plane, of a line joining the path of the beam to that detector which produces the output d,,.

2. Electron beam apparatus comprising means for causing an electron beam to impinge on the surface of a specimen placed in the path of said beam, an array of n electron detectors, n being at least three, spaced symmetrically around the path of said beam in positions to pick up resultant backscattered electrons from said specimen surface, said detectors producing electrical signals dependent on the backscattered electrons picked up, means for causing relative lateral scanning between said beam and said specimen surface, and an electric mixing circuit, said mixing circuit being arranged to produce an output *=r -d me-di+i+ fl+ where d d, d are the electrical signals from said detectors and d'=l/n(d,,+d,+ d,,,,).

3. Electron beam apparatus comprising means for causing an electron beam to impinge on the surface of a specimen placed in the path of said beam, an array of n electron detectors, n being at least three, spaced symmetrically around the path of said beam in positions to pick up resultant backscattered electrons from said specimen surface, said detectors producing electrical signals dependent of the backscattered electrons picked up, means for causing relative lateral scanning between said beam and said specimen surface, and an electric mixing circuit, said mixing circuit being arranged to produce an output D"=(d-d,,) +(dd,)+ where d (1, d are the electric signals from said detectors and d'=l/n(d,,+d,+ d

4. The apparatus ofclaim 1 including means associated with said mixing circuit for varying the value ofthe angle 0.

5. The apparatus of claim 1 including switching means associated with said mixing circuit to allow selection at will of a signal representing the sum of the outputs of all said detectors.

6, The apparatus of claim 1 further including switching means associated with said mixing circuit and allowing at will the selection of a signal D=|dd I+[d d [+]d d where d'=l/n(d,,+d,+ +d,,,,).

7. The apparatus of claim 1 further including switching means associated with said mixing circuit and allowing the selection at will of a signal:

8. The apparatus of claim 2 further including switching means associated with said mixing circuit and allowing the selection at will ofa signal:

9. A method of determining the topography of the surface of a specimen comprising causing an electron beam to impinge on said surface, detecting the resultant electrons back scattered from said surface by means of an array of n separate detectors, n being at least three, symmetrically spaced around the path of said beam, causing relative lateral scanning between said beam and said specimen surface, deriving from the detectors at least three corresponding electrical signals and deriving from said signals an output signal 360 COS 3 10. A method of determining the topography of the surface of a specimen comprising causing an electron beam to impinge on said surface, detecting thevresultant electrons back scattered from said surface by means of an array of n separate detectors, n being at least three, symmetrically spaced around the path of said beam. causing relative lateral scanning between said beam and said specimen surface, deriving from the detectors at least three corresponding electrical signals and deriving from said signals an output signal id tidili j i liltijidi where d,,, zfilif fare the electrical signals from said detectors and d'=l /n(d,,+d,+...d,,,,).

II. A method of determining the topography of the surface ofa specimen comprising causing an electron beam to impinge on said surface, detecting the resultant electrons back scattered from said surface by means of an array of n separate detectors, n being at least three, symmetrically spaced around the path of said beam, causing relative lateral scanning between said beam and said specimen surface, deriving from the detectors at least three corresponding electrical signals and deriving from said signals an output signal D"=(d d,,)+(dd1) where d 1',...d,,, are the electric signals from said detectors and d=l/n(d,,+d,+...d,, 

1. Electron beam apparatus comprising means for causing an electron beam to impinge on the surface of a specimen placed in the path of said beam, an array of n electron detectors, n being at least three, spaced symmetrically around the path of said beam in positions to pick up resultant backscattered electrons from said specimen surface, said detectors producing electrical signals dependent on the backscattered electrons picked up, means for causing relative lateral scanning between said beam and said specimen surface, and an electric mixing circuit, said mixing circuit being arranged to produce an output where do, d1 ... dn 1 are the electric signals from said detectors, and theta is the angle between an arbitrary direction lying in a plane normal to the beam and the projection, onto said plane, of a line joining the path of the beam to that detector which produces the output do.
 2. Electron beam apparatus comprising means for causing an electron beam to impinge on the surface of a specimen placed in the path of said beam, an array of n electron detectors, n being at least three, spaced symmetrically around the path of said beam in positions to pick up resultant backscattered electrons from said specimen surface, said detectors producing electrical signals dependent on the backscattered electrons picked up, means for causing relative lateral scanning between said beam and said specimen surface, and an electric mixing circuit, said mixing circuit being arranged to produce an output D1 d1-do + d1-d1 + d1-d2 +... where do, d1 ... dn 1 are the electrical signals from said detectors and d1 1/n(do+d1+ ... dn 1).
 3. Electron beam apparatus comprising means for causing an electron beam to impinge on the surface of a specimen placed in the path of said beam, an array of n electron detectors, n being at least three, spaced symmetrically around the path of said beam in positions to pick up resultant backscattered electrons from said specimen surface, said detectors producing electrical signals dependent of the backscattered electrons picked up, means for causing relative lateral scanning between said beam and said specimen surface, and an electric mixing circuit, said mixing circuit being arranged to produce an output D11 (d1-do)2+(d1-d1)2+ ... where do, d1 ... dn 1 are the electric signals from said detectors and d1 1/n(do+d1+ ... dn 1).
 4. The apparatus of claim 1 including means associated with said mixing circuit for varying the value of the angle theta .
 5. The apparatus of claim 1 including switching means associated with said mixing circuit to allow selection at will of a signal representing the sum of the outputs of all said detectors.
 6. The apparatus of claim 1 further including switching means associated with said mixing circuit and allowing at will the selection of a signal D1 d1-do + d1-d1 +d1-d2 + ... where d1 1/n(do+d1+ ... +dn 1).
 7. The apparatus of claim 1 further including switching means associated with said mixing circuit and allowing the selection at will of a signal: D11 (d1-do)2+(d1-d1)2+ ... where d1 1/n(do+d1+ ... +dn 1).
 8. The apparatus of claim 2 further including switching means associated with said mixing circuit and allowing the selection at will of a signal: D11 (d1-do)2+(d1-d1)2+ ...
 9. A method of determining the topography of the surface of a specimen comprising causing an electron beam to impinge on said surface, detecting the resultant electrons back scattered from said surface by means of an array of n separate detectors, n being at least three, symmetrically spaced around the path of said beam, causing relative lateral scanning between said beam and said specimen surface, deriving from the detectors at least three corresponding electrical signals and deriving from said signals an output signal where do, d1...dn 1 are the electric signals from said detectors, and is the angle between an arbitrary direction lying in a plane normal to the beam and the projection, onto said plane, of a line joining the path of the beam to that detector which produces the output do.
 10. A method of determining the topography of the surface of a specimen comprising causing an electron beam to impinge on said surface, detecting the resultant electrons back scattered from said surface by means of an array of n separate detectors, n being at least three, symmetrically spaced around the path of said beam, causing relative lateral scanning between said beam and said specimen surface, deriving from the detectors at least three corresponding electrical signals and deriving from said signals an output signal D1 d1-do + d1-d1 +d1-d2 +... where do, d1...dn 1 are the electrical signals from said detectors and d1 1/n(do+d1+...dn 1).
 11. A method of determining the topography of the surface of a specimen comprising causing an electron beam to impinge on said surface, detecting the resultant electrons back scattered from said surface by means of an array of n separate detectors, n being at least three, symmetrically spaced around the path of said beam, causing relative lateral scanning between said beam and said specimen surface, deriving from the detectors at least three corresponding electrical signals and deRiving from said signals an output signal D11 (d1-do)2+(d1-d1)2+... where do, d1...dn 1 are the electric signals from said detectors and d1 1/n(do+d1+...dn 1). 